6.4.2 Obsolete Macros

Although using some of the following macros was required in past
releases, you should not use any of them in new code. All
these macros will be removed in the next major Automake version;
if you are still using them, running autoupdate should
adjust your configure.ac automatically (see Using autoupdate to Modernize
configure.ac in The Autoconf Manual).
Do it NOW!

AM_PROG_MKDIR_P

From Automake 1.8 to 1.9.6 this macro used to define the output
variable mkdir_p to one of mkdir -p, install-sh
-d, or mkinstalldirs.

Nowadays Autoconf provides a similar functionality with
AC_PROG_MKDIR_P (see Particular
Program Checks in The Autoconf Manual), however this defines
the output variable MKDIR_P instead. In case you are still
using the AM_PROG_MKDIR_P macro in your configure.ac,
or its provided variable $(mkdir_p) in your Makefile.am,
you are advised to switch ASAP to the more modern Autoconf-provided
interface instead; both the macro and the variable might be removed
in a future major Automake release.